1. Field of the Invention
The present invention relates to a paper making elastic belt used for pressing and dehydrating wet web in a field of a paper industry and the like, and more particularly, relates to a paper making elastic belt having a drainage channels in its surface.
2. Description of the Background Art
General required characteristics for the paper making elastic belt such as a shoe press belt includes strength, crack resistance, abrasion resistance, flexibility, impermeability to water, oil, gas and the like. As a material comprising the above characteristics, polyurethane provided by reacting urethane prepolymer with a curing agent is used in general.
In a paper making technique, it is known that many drainage channels are provided in an outer surface of the elastic belt along a travel direction of the wet web in order to drain water extracted from pressed wet web. For example, U.S. Pat. No. 4,559,258 discloses an elastic belt of a paper making machine comprising such drainage channels.
FIG. 4 is a sectional view showing a paper making elastic belt 1 comprising many drainage channels 2. Conventionally, in order to avoid leaving traces of the channels on the wet web, a channel width of the paper making elastic belt 1 is made small. The channel width is 0.6 to 1.5 mm in general.
Other required characteristics for the paper making elastic belt 1 having drainage channels 2 in its surface includes a drainage property in which water extracted from the wet web is immediately drained from the channel 2 of the belt 1 to the outside. If the water in the channel 2 is not drained during one rotation of the belt 1 which is rotating at high speed, the wet web becomes wet again and a property of squeezing out water from the wet web is lowered.
As factors which aggravate the drainage property, low drainage performance of the channel of the belt itself, low drainage performance due to attachment of a paper residue, and a small gap of the channel due to abrasion or compressive strain of the channel while it is used are considered. Japanese Unexamined Patent Publication No. 2002-220789 focused on such problems. The above document discloses a shoe press belt in which a surface of a wet web side layer is made hydrophobic with a fluoro resin, a silicone resin and the like.
According to a constitution disclosed in the above document, the drainage performance of the belt itself can be improved by forming the surface of the belt with the hydrophobic material. However, prevention of attachment of the paper residue cannot be improved. In addition, as disclosed in the above document, when the surface of the wet web side layer is formed of the hydrophobic material, the above general required characteristics, that is, the strength, the crack resistance, the flexibility and the impermeability could be inferior to the case polyurethane is used, so that many issues to be solved are left in practical utilization.
The inventor of the present invention focused on surface roughness of a wall of the drainage channel as a factor to improve squeezing performance of the paper making elastic belt. Those skilled in the art have not focused on the surface roughness of the drainage channel so far.
Since a channel width is as small as 0.6 to 1.5 mm when the channel is cut and shaped in the elastic belt, a cooling process with cooling water at the processing part becomes unstable or a swarf cannot be smoothly discharged, so that the conventional surface roughness of the channel wall is 3 to 4 μm as hard as possible. The inventor of the present invention found that when the surface roughness of the channel wall became a predetermined value or more, resistance to water flow became great or paper residue was likely to be attached, so that the squeezing performance of the elastic belt was lowered.